What is quench?

[DaGr8Tim]

I know this is a noobish question, but I've searched the net several times and have not recieved an answer that fully explains it. I know it has something to do with how high the piston comes or maybe the amount of space with piston at TDC, but I really do not understand it.

What's got me wonding is a friend asked me to take some high res pics of my HSO heads for comparision with other 2.3 HSC & HSO heads.

 

[rickwrench]

Quench (aka squish and squench) is an area between the flat face of a piston and the flat face of the head. As the piston nears TDC these two surfaces meet and smash the A/F mixture between them, squirting it out at high velocity into the combustion chamber. This does two things, first, it cools hotspots on the piston and cylinder head, and, more importantly, it creates a huge amount of turbulence and tumble in the chamber, which thoroughly mixes A/F for an even flame front (= no ping).
A head with a large quench surface can run higher compression, advance, and worse gas than a non-quench head, with no problems.
Ideal quench surfaces meet at .035"-.040" of clearance between piston and head. Anything more than .055"-.060" and you might as well be running open chamber heads.
A zero deck 200 with an .040" gasket is PERFECT. A stock 250 is a bad joke, with the pistons sitting so far down the hole.
Rick(wrench)

 

[80broncoman]

Others on here will probibly have a better definition, but here is my understanding of it.
When a piston is at TDC, It is the area of the piston that will be the closest to the head. This does not count the dome if any or velve eybrows.
As the piston is rising (on the compression stroke) just before it reaches the
End of its upward travel the mixture in the area of quench gets squezed out of the quench area very quickly . This causes a violent mixing of quench area
mix and non-quench mix and you get a better burn.
Hope it makes sense.

EDIT I was a little slow to the keyboard.

 

[Anonymous]

Both of these assessments are correct, but there is an additional feature of quench not mentioned. As stated, quench is the phenomenon that occurs where fuel/air mix gets squished between the flat surface of the piston and the flat surface of the cylinder head as the piston approaches TDC. When this occurs the fuel/air mix is violently blown toward the larger volume of the combustion chamber. And as stated, this violent occurrance causes the fuel/air to mix better giving a more complete burn.

Now keep in mind that as the piston approaches TDC, the timing advance initiates spark several degrees before the piston reaches TDC where the quench effect takes place. What this means is that there is already a flamefront propigating from it's initiation point at the spark plug into the rest of the combustion chamber before the quench effect happens. Imagine throwing a stone in a pond and the wave created,,, this is how the flame front propigates outward.

The flamefront in an engine without quench would burn until it reached the boundaries of the combustion chamber in each direction and would then extinguish. This flametravel takes a relatively constant amount of time to completely burn the fuel/air mix. But as rpm's increase, the piston is moving much faster through the power stroke. So in order to get all the fuel/air mix burned in time,, we have to keep starting the flamefront ignition earlier and earlier to get max power. Advancing the timing usually does improve power output,, but think of what it actually does. If you're lighting your fuel/air mix before TDC, you are actually building pressure that is trying to force the piston down the cylinder backwards. So though advance is necessary at higher RPM's in order to get your fuel/air mix burned in time,, it also decreases efficiency as some of the pressure is working against itself.

Here comes the missed point. An engine with quench can completely burn the fuel/air mix in a much shorter amount of time, and here's why. Remember the flame-front traveling outward at constant speed?? Remember also the fuel/air mix violently rushing inward towards the center of the combustion chamber? Well what's happening is the fuel/air mix is getting blown into the flamefront which quickly burns the entire fuel/air mix in much less time than it would take if the flame had to travel all the way to the boundarys of the combustion chamber. This is a huge part of the fast-burn effect you hear about.

The faster the burn, the less timing advance you have to run to get the fuel/air mix burned in time. The less timing advance, the less pressure that's trying to force the piston down the cylinder backward,, and the more power your engine will make. This fast burn effect is also the reason you can run higher compression. You can run higher compression because the fuel air mix has a lot less time to get hot and detonate because it gets burned so quickly. Higher compression ratio increases power and efficiency of your engine.

 

[Lazy JW]

jgetti,
Thanks for posting that info, it clears up the quench theory quite well, at least to me. Sounds like an EFI head/piston combo with zero deck is probably going to deliver best economy.
Joe

 

[AzCoupe]

Here's a good article on quench.
http://chevyhiperformance.com/techarticles/94138/

 

[FloridaRustang]

So based on Rick's assessment of the 250 block, how much milling of the block surface should/could you do to help the situation?

 

[rickwrench]

A stock US 250's pistons have a 5cc dish and are .134" down in the hole. The heads have a 62cc chamber.
This gives a C/R of 8.2-1 with the original gasket, 7.8/7.9-1 with a replacement composite gasket.
Seems like most people use flattop ford 255 pistons that are .085" taller in compression height than the stock pistons (1.585" vs 1.5"). This gets you to 9.6-1 C/R with a composite gasket, but with no quench effect at all (ping).
There was a discussion a few weeks ago about gasket thicknesses and their availability. Before you mill the block, get the right thickness gaskets in hand.
Once you get to a good (between .035" - .040") quench you can have the 255 pistons dished to get to an acceptable C/R.
Rick

 

[ASMART]

It would seem that to keep the quench going the dishing would need to be done to mirror the "dish" in the head.

 

[Lazy JW]

It would seem that to keep the quench going the dishing would need to be done to mirror the "dish" in the head.

There may well be some advantage but considering that Sir Harry did significant pioneering work with quench on flatheads it seems evident that it is not absolutely necessary.
Joe

 

[CZLN6]

Howdy All:

Floridarustang- I decked the top of my 250 block .070" and felt that was as far as I could safely go, for structural reasons. It is not enough, by itself to obtain a good quench dimension.

If you were to add to this the taller 255 V8 pistons, you'd be pretty close to zero. Another option I explored several months ago is as follows-
********************


"Use a longer Tempo 2.5 HSC rod with the flattop HSC piston. This has the advantage of reducing the deck height .120" and giving a slightly better rod/stroke ratio.
Rod Length & weight;
250 =5.88" 592 grams
2.5 HSC =5.99" ??? grams
Difference = .110" grams

Rod Length to Stroke ratio;
250 = 5.88" : 3.91"= 1.5:1
2.5 HSC = 5.99" : 3.91"= 1.5319:1

Deck the top of the block to achieve zero deck height. Use a FoMoCo composite head gasket with a compressed thickness of .037".

Use with an HSC flattop piston, with plans to mill a "D" shaped dish into the top. The goals will be to create a higher quench to bore ratio, lower CR, lighten the piston, reduce knock tendency, and maximize combustion efficiency.

Mill the head only enough to ensure it's flat. At this point it is critical to measure all volumes to assess CR. Reshaping the combustion chambers to reduce CR, unshroud the intake valves and match chamber volumes is critical.

The advantages of using the 4-cyl rods are- longer, for a slightly improved rod to stroke ratio, tougher than I6 gear. The four cylinders vibrate much more, rev to a much higher RPM, produce more power per cylinder, carry more load per cylinder than a six, and suffer more detonation than a I6 was ever designed for.

This combination will have a more ideal deck clearance, likely have a lighter reciprocating weight, and be able to tolerate CR in the 9.5:1 range, at sea level, with 91 octane gas.

Additionally, plans are to use ARP rod bolts, polish the rod beams, balance the whole rotating assembly, and polish the piston tops and the chambers."
************************

Optimizing quench is a worthy goal- however you get there.

Finding 6 2.5 HSC rods is a challenge.

Adios, David

 

[JackFish]

If you were to add to this the taller 255 Vee-Eight pistons, you'd be pretty close to zero. Another option I explored several months ago is as follows-
********************
"Use a longer Tempo 2.5 HSC rod with the flattop HSC piston. This has the

Can the taller pistons and/or the rods be used in a stock 200?

 

[CZLN6]

Howdy Back All:

Jackfish- No. The typically deck height on a 200 block is about .025". The 255 V8 pistons are .085" taller than stock 200 pistons. That would have them sticking out of the block about .060"

The HSC 2.5 rod is about measure 5.99" compared to a 200 rod at 4.715". that's over an inch and a quarter taller. NO WAY!!!

I know- we gotta keep looking, but this is only an option, if you can find them, for a 250s deck height hole.

Merry Christmas,

Adios, David

 

[Anonymous]

I would worry about any single answer saying how far to deck YOUR block to get to a particular squish-height.
Block dimensions vary, piston compression heights (from pin to top) can vary, rod lengths can vary, crankpin stroke can vary. Sometimes the crankpins are not evenly "clocked." If you want to see all of these examples in their full, sloppy glory, try blueprinting an older Mopar!! I think you need to take measurements.
If you are doing a full rebuild, you can mix and match short and long components to get to fairly even assembled heights. And I don't know that you can avoid tearing down the engine, because your machinist might balk at putting an assembled shortblock on his machinery just to mill the deck. In any case, I would Think (now, correct me if I'm wrong) that you need to ascertain the amount of material you want to take off the deck by an ancient hot-rodders technique called claying the pistons.
Pull the head and clean things up. Take a small pieces of modelling clay and roll them out into cylinders about 1/8-3/16" diameter. Affix these to the squish areas of the head, about 1/8" in from the outer edge. With your fingertip, wipe a little oil on the piston edges so the won't stick to the clay. Very carefully, with a helper, set the head on the block, with gasket, and torque it fairly close to the final torque specs. Manually turn the engine over one full turn.
Take the head off, carefully, and set it upside down. Take a razor blade and cut through the middle of one of your squished clay cylinders. Peal off half of the clay. What remains on the head is a cross-section of clay that you can very gingerly measure with the depth probe on the back end of your dial calipers. Do this on all cylinders, and write it down.
The smallest reading, which shows that piston being closest to the head, is the reading you have to work with. Subtract .042" if you are conservative, or .035" if you are bold, from that reading, and the remainder is the amount that needs to be milled off your block.

That's how I do it, but I taught myself, so maybe there are better ways.

 

[Stubby]

I thought it was that first cold drink on a hot day.

 

[CZLN6]

Howdy Back All:

Seattle Smitty- The problem isn't determining how much you need to mill off the top of the block to get to zero deck height. The problem with the 250 is structural rigidity and strength. My 250 had the pistons .150" below the deck. After taking .070" off it became clear, by peering into the water jackets, that anymore off the deck would compromise a solid deck surface.

All- The small diameter bore of the 200/250 is another positive factor in combustion efficiency. The smaller the bore the less distance the flame front must travel before it reachs a boundary. That's not including the wedge combustion chamber and the dished pistons. They each add to combustion efficiency too.

Ideally, a zero deck height, a small combustion chamber, with a "D" shaped dish in the piston that mirrors the chamber, and a head gasket with a .035" compressed height, would be the goal to an efficient engine.

Unfortunately, the thinnest available head gasket is the Victor (NAPA) at .044" to .047", there are no pistons currently available with a "D" shaped dish that mirror the combustion chambers, and the chambers are already too small for good flow due to valve shrouding.

Fortunately, zero deck height is a reasonable option on a 200, small dish pistons are readily available, and some reshaping of the chambers to reduce valve shrouding is possible without ruining quench.

Another option to reduce deck height, not often mentions, is offset grinding the crank journals. If your crank will allow and your machinist is capable he can offset the cut when the crank journals are being ground to under sizes. This would move the rod away from the centerline of the crank say, .010" for an increase of .020" in stroke- 3.126" to 3.146", and reduces deck height by .010".

As I've said before, there are options and solutions yet to be discovered. We gotta keep looking.

Adios, David

 

[wsa111]

David, your recomenditions are very informative, I do not have a 250 but thinking ahead with Mikes new cylinder head, what combustion size would be ok for the 200-50 cc's,but what size would you need with the 250 engine???

You need to give Mike feedback so both applications can be addressed, William

 

[Anonymous]

Got it, Dave; thanks.

 

[optikal illushun]

Relating to quench, how does this effect flame promligation (sp)? or is this the aforementioned flame front?

i asked my diesel engines teacher about quench in a diesel and is it like in a gas engine and he was amazed i knew anything about that

 

[Anonymous]

Gentlemen;

There is also one other important point to the "quench" of a wedge shaped combustion chamber. Consider the Bunsen burner, your hand-held propane torch or an aircraft jet engine with afterburner. Besides the benefit of forced turbulent rapid mixing and burning, there is also the important need for flame stabilization or combustion stability.

Combustion stability can be, and is, a technically detailed topic to discuss.
So consider the burners mentioned above. If one moves the combustion gas too fast through the Bunsen or propane or afterburner, the flame simply "blows-out." Much like blowing out a candle. Thus, it is easy to recognize that flames need to be stabilized. Clearly, for 4-stroke IC engines, slow buring flames rob the engine of power. However, extremely fast burning flames can lead to detonation (or supersonic combustion). A combustion wave or flame that is subsonic is a deflagration ... for supersonic it is a detonation ... and for the exact case of sonic we have the rare occurance of a Mach wave (named after Ernst Mach of famed Mach number ... like Mach 1).

The "quench" chamber head helps to stabilize a "deflagration combustion" by allowing the formation of a viscous boundary layer between the gas and the cylider head/piston. In this boundary layer formation is the ability to allow large amounts of heat-transfer (and slower moving gas) to cool the gas and preclude the onset of supersonic combustion (ie pinging). As the theory goes, hi temp combustion gas is prone to the occurance of short "detonation inductance distance" (ie DID). This is a measure of the combustion condition required to transition from subsonic to supersonic combustion. The higher the temp, the shorter the DID, the faster the onset of shock waves (supersonic combustion) and pinging (very bad for IC engines). Also, increased cylinder pressure (like that found in hi compression motors) only adds to the probability of inducing detonation.

The viscous boundary layer, between the head/piston, also allows the short-lived formation of a flame-holder effect like the burners mentioned above. Thus adding to flame stabilization. Just one more aspect of the "quench effect" for a wedge shaped cylinder head.